Composition for screening infra-red radiation

ABSTRACT

A composition for nullifying detection with infra-red equipment consistingssentially of a suspension of 30-40% by weight of aluminum particles in from 60 to 70% by weight of vaporizable hydrocarbon oil, said aluminum particles have diameters in range of 2 to 20 microns and thickness of less than 3/10 micron.

This application is a division of my prior application Ser. No. 35,380filed June 10, 1960.

This invention relates to screening infra-red radiation. It is useful,e.g., in frustrating reconnaissance by infra-red photography or blankingout detection apparatus employing infra-red radiation.

In the drawings, FIGS. 1 and 2 are diagrammatic illustrations ofapparatus for carrying out the invention.

The method comprises forming between the source of infra-red radiationand the point of reception an aerosol or "smoke screen" composed ofsmall flat flakes of material which is opaque to the infra-red radiationand of high respectivity, the maximum dimensions of the particles beingof the same order of magnitude as the wave lengths of the radiation,aluminum flakes having diameters in the range of 2 to 20 microns andthicknesses of less than 3/10 micron. (The minimum thickness must besufficient to render the particles opaque. This is so far below thethickness of available particles as to be of no particularsignificance.) Aluminum powder of this character is commerciallyavailable, being used as pigment. A suitable concentration is about 1.15mg. per cubic foot of air. Thus, a cloud 500 feet wide, 1000 feet longand 11 feet deep would contain about 75 Kg. or 165 lbs. aluminum. Thismay, however, be widely varied, since the effectiveness of screening isa function of the concentration and length of path through the aerosol.The concentration and size of the cloud will therefore be specified onthe basis of various factors, among them strength of the radiation fromthe target or source, the distance between the detector and the targetor source, the type of detecting means employed.

There is one important factor which does not affect the requiredconcentration and the cloud size, namely, the wave-length of theradiation. This is one of the great advantages of my method as comparedto the use of the usual oil fog "smoke" screens. The latter are veryeffective against the "near" infra-red radiation of less than about onemicron wave-length but fall off very rapidly in effectiveness as thewave length increases, becoming almost completely ineffective at wavelengths of about 5 microns. In contrast, the aluminum flake "smoke"screen gives almost constant per cent attenuation across the wave-lengthspectrum up to at least 15 microns wave length.

The effectiveness of the agent may be expressed in terms of the "JValue", defined by the formula J=ln (Io/I_(T) /LC)), wherein: Io is theintensity of the radiation received at the receiver in the absence of"smoke", I_(T) is the intensity of the radiation under the sameconditions through the smoke, L is the length of the path through thesmoke, and C is the concentration of the agent in the cloud. The unitsused for Io and I_(T) are immaterial, so long as they are the same. C ispreferably expressed in weight per unit volume, the units of C and Lbeing consistent. For example, C is desirably expressed in grams percubic meter and L in meters. In that case, J will be expressed in squaremeters per gram of agent.

A simple physical expression of the meaning of the "J Value" is asfollows. Any particle exposed to radiation to which it is opaque willcast a shadow. Then the dimensions of the particle approximate the wavelength of the radiation, the area of the shadow, considering a radiatorat infinity or otherwise giving parallel rays, is greater than the crosssectional area of the particle. This shadow area is termed the"scattering cross section" of the particle. The "J. Value" is the totalscattering cross section of the particles per unit weight, with theparticles dispersed and having random orientations.

We have found that the "J. Value" of the aluminum flakes is considerablygreater than the average total area of the flakes in any one direction,assuming random orientation and also represents an appreciable increaseover the theoretical "J. Value" at long wave lengths obtained forspherical metallic particles, where the ratio of mass to scatteringcross sectional area increases as the particle size increases. In fact,it is nearly equal to the total area of the flakes in the plane of theirfaces as measured by standard "covering area" tests.

The aluminum powder may be disseminated either as a dry powder or byintroducing a slurry of oil and powder into a hot, high velocity gasstream in which the oil is atomized and vaporized.

A slurry of 30% to 40% by weight of the aluminum powder in 70% to 60% byweight of the standard fog oil, a hydrocarbon oil employed in producingoil fog smoke screens and described in U.S. Government SpecificationMil-F-12070A, is satisfactory. Because of the light, fluffy nature ofthe dry powder, such a slurry contains a greater weight of powder perunit volume than does the dry material.

For the dissemination of this aluminum powder, the dry powder or theslurry is preferably introduced into a hot, high velocity gas stream.FIGS. 1 and 2 of the drawing show, diagrammatically, suitablearrangements for the handling of the dry powder and the slurry,respectively. I have found that a gas turbine is a very suitable andconvenient source of the gas stream.

For example, I have used successfully the "Mars" turbine, manufacturedby the Solar Aircraft Company. This is a single stage turbine includinga directly corrected single stage turbine compressor for supplying airto the combustion chamber. Hydrocarbon oils are used as fuels. When usedto deliver powder it develops 45 hp at 40,000 r.p.m. When used foraerosol generation the power takeoff is removed and a 3 inch or 4 inchdiameter venturi is attached to the exhaust. Temperatures in the rangeof 500° F to 1000° F and gas velocities up to about 1000 ft./sec. areobtained. The gas velocity and temperature are controlled by adjustmentof the fuel supply to the turbine and the choice of the venturi size.

Referring to FIG. 1, the gas turbine unit is indicated at 1. A venturi 3is connected to the exhaust of the turbine. An air line 5 bleeds offcompressed air from the compressor stage of the turbine and leads it toan air injector 9 located at the bottom of the closed container 7 inwhich the dry aluminum powder is stored. The air line 5 contains asolenoid valve 8 and check valve 10. The powder in container 7 isfluidized by the air injector through valve 8 and conveyed throughsupply line 11 to the throat venturi 3. Supply line 11 is controlled bya globe valve 13.

The operations of this system is as follows. A high velocity gas streamissuing from the exhaust of turbine 1 through venturi 3 entrains thepowder introduced through supply line 11 and disperses it rapidly intothe atmosphere. Because of the very fine character of the powder thecloud which is produced is quite stable. It serves as the "smoke"screen. In this system, employing the dry powder, it is not necessarythat the gas stream be hot. However, we have found the gas turbine to bea very compact and efficient means of producing the high velocity gasstream required.

The system for dispersing the powder through the use of an oil slurrywill be described next. This system is shown in FIG. 2, in which thesame numerals indicated the same elements as in FIG. 1. The onlydifference is that in the slurry system it is unnecessary to fluidizethe material since the agent is a fluid. A suspension or slurry of thealuminum powder in fog-oil is stored in a closed container 15. Supplyline 11 is provided with a lower portion 17 within the container 15. Airline 5 is connected to the upper part of container 15. The air deliveredthrough air line 5 forces the suspension through the supply line 11 toventuri 3.

The turbine exhaust gases flowing at a very high velocity throughventuri 3 atomize the oil-powder suspension and because of the hightemperature vaporize the oil. The mixture of aluminum powder, fog-oilvapor and hot gases is discharged into the atmosphere. The fog-oil thencondenses to form liquid aerosol particles in the same manner as in theusual fog-oil "smoke" generation. The aluminum particles are at the sametime thoroughly dispersed into the atmosphere in the form of a cloud.Apparently the aluminum particles are ordinarily separated from thefog-oil particles. At any rate, the fog-oil aerosol in no way interfereswith the screening action of the aluminum flakes.

The following examples illustrate my invention.

EXAMPLE 1

This test was conducted in a closed chamber. An infra-red source and adetector were so mounted as to give a path length between then of 14.5meters. The chamber was equipped with electrostatic samplers whichdetermined the concentration of screening agent per unit volume. A"Mars" gas turbine engine manufactured by the Solar Aircraft Companywith a venturi fitted over its exhaust was arranged to generate anaerosol within the chamber. The agent (dry aluminum powder or fog oil)was introduced into the throat of the venturi.

The materials used were Reynolds 40XD aluminum powder and fog oil. The40XD powder had a 1% retention on a 325 mesh screen. By microscopicmeasurements, we found it to have the following size distribution.

    ______________________________________                                        MAXIMUM DIAMETER OF PARTICLES                                                                        AREA, PER CENT                                         IN MICRONS             OF TOTAL                                               ______________________________________                                        0 - 5                  24.2                                                    5 - 10                39.2                                                   10 - 15                19.1                                                   15 - 20                17.4                                                   ______________________________________                                    

The areas given above are those taken in the planes of the faces of theflake-like particles. By means of electron microscope photographs wehave found a typical sample to have thicknesses as follows:

    ______________________________________                                        CUMULATIVE       THICKNESSES IN MICRONS                                       PER CENT OF PARTICLES                                                                          LESS THAN                                                    ______________________________________                                        71               0.15                                                         81               0.20                                                         95               0.40                                                         97               0.45                                                         ______________________________________                                    

The total covering area of the flakes as measured by surface coveragetests (See "Edwards" Aluminum Paint and Powder, published 1936 byReinhold Publishing Corp., pages 36-41) was 3.2 square meters gram andthe total average area in any one direction assuming random orientationis shown mathematically to be 0.405 times that area or 1.3 square metersgram.

Sufficient aluminum powder was introduced to give a concentration of 100micrograms/liter. Comparison tests were conducted using fog oil "smoke"to insure a controlled standard. From a large number of runs, thefollowing data was obtained.

    ______________________________________                                                            J VALUE IN                                                                   M.sup.2 /GRAM                                              WAVELENGTH OF RADIATION                                                                            Aluminum  Fog Oil                                        IN MICRONS           Flakes    "Smoke"                                        ______________________________________                                        0.75                 2.3       2.8                                            1.0                  2.3       1.6                                            1.5                  2.3       0.8                                            3.0                  2.4       0.1                                            6.0                  2.4       0.05                                           12.0                 2.5       0.03                                           ______________________________________                                    

EXAMPLE 2

A field test was conducted to compare the screening effects of aerosolsof (a) fog oil, and (b) a mixture of 40% by weight flake aluminum powderand 60% by weight fog oil. The flake aluminum powder was the sameproduct as employed in Example 1.

A target and a detector were mounted about 800 meters apart. Thedetector consisted of a modified "Penrod" unit which utilizes athermistor bolometer as the sensitive element. This unit was placed on a60 ft. tower to insure a good field of coverage. The modificationsconsisted in taking the signal off the third amplification stage andthen rectifying and feeding the signal to an L & W recorder. The targetwas a cut down oil drum equipped with a stack and a draft. This drum waspartially filled with oil which was ignited during the test. Thetemperatures of the target were 230° for the drum and 420° F. for thestack. Using these temperatures and assuming an emissivity of one, a 60%atmospheric attenuation and an 800 meter range, the target signal at thedetector was of the order of 6 × 10⁻⁸ watts per cm².

The gas turbine aerosol generator previously described was used, thearrangement shown in FIG. 2 of the drawing being employed. The generatorwas positioned between the target and detector, about 300 meters fromthe target and about 75 meters off the direct line between the targetand detector on the windward side of that line.

Each run was of one minute duration. During the run employing fog oilalone 18 lbs. of the agent was employed. On successive runs, 11 and111/4 lbs. of the aluminum-fog oil mixture was used. The wind velocitywas between 5 and 7 m.p.h. and the cloud height was estimated at 50 ft.The cloud was not homogenous because of wind variations and terrainfeatures.

The radiation received by the detector was continuously recorded.Because of the uncertainty and variation inherent in the fieldconditions, quantitative values such as those given in Example 1 couldnot be obtained. Qualitatively, the aluminum-fog oil mixture gave anattenuation of the radiation many times greater than that given by thefog oil alone. At the optimum points of each run, the attentuation bythe mixture approached 100%.

I claim:
 1. A composition for use in generating a protective aerosol,consisting essentially of a suspension of 30 to 40% by weight ofaluminum particles in from 60 to 70% by weight of vaporizablehydrocarbon oil, said aluminum particles having diameters in the rangeof 2 to 20 microns and thicknesses of less than 3/10 micron.